What they are looking at are not dead horseshoe crabs but their molts that they need to shed at least once a year. In areas where Limulus is common, the shells, exoskeletons or exuviae (molted shells) of horseshoe crabs frequently wash up on beaches, either as whole shells, or as disarticulated pieces.

The shell of these animals consists of three parts.

The carapace is the smooth frontmost part of the crab which contains the eyes (five pairs), one pair of small pincers/chelicerae used to move food towards the mouth, five pairs of walking legs (the first four with claws, the last with a leaflike structure used for pushing), the mouth in between the legs, the brain, and the heart.

The abdomen is the middle portion where the gills are attached as well as the genital operculum. The last section is the telson (i.e., tail or caudal spine) which is used to steer in the water and also to flip itself over if stuck upside down.

The horseshoe crab can grow up to 60 centimetres (24 in) in length (including tail); the female is typically 25 to 30 percent larger than the male.